Confusing speaker specifications

Discussion in 'General Electronics Chat' started by Gump, Nov 26, 2011.

  1. Gump

    Thread Starter Member

    Jun 7, 2010
    57
    1
    Hello,

    I have bought these speakers for a small project, but I'm struggling with the technical details - particularly how much voltage is actually needed and how much current is drawn.

    1) The speakers require 4 AA batteries, but can use instead a 5V power adapter. So, is it more likely that the speakers actually only needs 5 volts rather than the 6 volts provided by the batteries and is probably using a voltage regulator for the batteries?

    2) The rated input power is 3 watts, so wouldn't this mean that the current is 0.6 amps (I=P/V = 3/5), if so, why have they stated a DC power adapter of 1 amp?

    Thank you.
    Gump.
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,432
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    The power drawn from the power supply or battery is not the same as the audio power delivered to the speakers.

    Most audio amplifiers do not use a voltage regulator because it is not needed. A voltage regulator would just waste electrical power.

    You can get a rough estimate of the delivered power to the speakers by assuming the full battery voltage is applied to the speaker. For this you need to know the impedance of the speaker. Use the RMS voltage.

    So if the voltage is 6V and the speaker is 4 ohms, the RMS wattage would be 3 x 3 / 2 / 4 = 1.1W.

    Now some manufacturers cheat and quote peak power instead = 3 x 3 / 4 = 2.2W.

    And remember to multiply by 2 for two speakers.

    The adapter has to exceed this power rating otherwise the signals will be clipped resulting in distortion.
    Most adapters for applications such as audio speakers do not require regulated output. A 5V adapter will usually have a no-load output as high as 7 or 8 volts.
     
    Last edited: Nov 26, 2011
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  3. Adjuster

    Well-Known Member

    Dec 26, 2010
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    I would very much doubt that the "6V" battery would require any regulation down to 5V. For a start, over much of its life the battery voltage will be less than 6V, running down to 5V or less before the battery is considered discharged. A practical battery amplifier therefore has to cope with this range of variation.

    As for the current drain, no amplifier is 100% efficient, so even on that basis it would be normal to expect the input power required to be greater than the output power.

    In addition, it is a good idea to allow for a "safety factor" or margin for error, so that you can be sure that the power supply will not be overloaded, and preferably will run at less than its full rating. This is likely to make it more reliable, and allow it a longer lifespan.
     
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  4. Gump

    Thread Starter Member

    Jun 7, 2010
    57
    1
    Hello,

    Thanks for the replies.

    Ah I'm sorry, I think I've confused things then. I'm actually struggling with how much current the speakers would pull from the batteries.

    Thanks,
    Gump.
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,432
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    It depends on how loud you are going to play your music. 2W + 2W is a lot of acoustic power. At normal listening level, 1/10 of that should do, 0.2W + 0.2W.

    So I am guessing about 100 - 200mA.
     
    Last edited: Nov 26, 2011
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  6. Gump

    Thread Starter Member

    Jun 7, 2010
    57
    1
    Hm, perhaps I've ordered speakers that are too big... Is it possible for me to work out the maximum possible current? I've made an attempt:

    Current = Power / Voltage
    0.5A = 3W / 6V

    The 3 watts is the Rated Input Power, and voltage is the full voltage of the batteries.

    Thank you.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    That's for a single-ended output. If the amp has a bridge output then the peak output voltage is 6V (assuming the amp has no voltage drop at the voltage peaks) and the RMS power would be 6 x 6 / 2 / 4 = 4.5W.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    If 3 watts is the input power and not the speaker power then your calculations are correct.

    The reason they state you should use a 1 amp adapter is likely just as a design margin.
     
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  9. Gump

    Thread Starter Member

    Jun 7, 2010
    57
    1
    If I knew only half of what you knew I'd be fine... Thanks very much. :)
     
  10. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Will you play continuous tones at full blast? Then you need a continuous input of 3W.

    Or do you play normal music that has occasional peaks but averages 1/10th the peak power? Then you need 3W for the duration of the peaks and only 300mW for most of the time.
     
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